1. Field of the Invention
The present invention relates to the design of polymer electrolyte membrane (PEM) water electrolysers for the production of hydrogen and oxygen gases, and more particularly, to a PEM water electrolyser module and components therefor.
2. Discussion
Electrolysers use electricity to transform reactant chemicals to desired product chemicals through electrochemical reactions, i.e., reactions that occur at electrodes that are in contact with an electrolyte. Hydrogen is a product chemical of increasing demand for use in chemical processes, and also potentially for use in hydrogen vehicles powered by hydrogen fuel cell engines or hydrogen internal combustion engines (or hybrid hydrogen vehicles, also partially powered by batteries). Electrolysers that can produce hydrogen include: water electrolysers, which produce hydrogen and oxygen from water and electricity; ammonia electrolysers, which produce hydrogen and nitrogen from ammonia and electricity; and, chlor-alkali electrolysers, which produce hydrogen, chlorine and caustic solution from brine and electricity.
Water electrolysers are the most common type of electrolyser used for production of gaseous hydrogen as the main product of electrolysis. Polymer electrolyte membrane (PEM) water electrolysers are coming into more common commercial use, at least at a small scale. PEM water electrolysers use polymer electrolyte membranes, typically with appropriately catalyzed electrodes deposited on either side to form membrane-electrode assemblies (MEA). Hydrogen is produced at the cathodes (negative electrodes), and oxygen is produced at the anodes (positive electrodes) upon passage of current between the electrodes. The rates of production of hydrogen and oxygen are proportional to the current flow in the absence of parasitic reactions for a given physical size of electrolyser. The reactions are as shown in equations (1)-(3):Cathode: 2H++2e−→H2  (1)Anode: H2O→½O2+2H++2e−  (2)Cell: H2O→H2+½O2  (3)The electrolyte consists of the hydrated proton exchange membranes, which are ionically (proton) conducting through migration of protons between ion exchange sites under a voltage gradient. The solid membranes also serve to maintain the hydrogen and oxygen gases separate and of high purity.
The scale of PEM water electrolysers has generally been limited to about 10 Nm3/h or less in commercial applications, even with multiple cell stacks. In general, PEM water electrolyser cell stacks remain limited in active cell area and the number of cells per cell stack. There have been only limited attempts to design and demonstrate scale up of PEM water electrolyser cell stacks.
Stucki et. al [J. Appl. Electrochem., 28 (1998) 1041-1049] reported testing of relatively long cell stacks with 120 cells and an active cell area of 400 cm2; significant durability and lifetime issues were reported. Durability and lifetime of PEM water electrolysers remain as an ongoing challenge, even with small sized cell stacks, particularly as relates to durability and lifetime of proton exchange membranes. The challenge tends to scale with the size/capacity of the cell stack, and is exacerbated by the trend toward use of thinner membranes to improve cell polarization performance. Hypothesized membrane failure modes include: (i) localized “hot spots”, due to high local current densities and/or insufficient cooling; (ii) mechanical stress on membranes due to operation with differential pressure across the membranes, leading to membrane creep, especially at typical target elevated operating temperatures of 80-90° C.; and, (iii) chemical attack on perfluorocarbon backbone of membranes by peroxide reaction intermediates. Approaches to addressing (iii) include the advent of “chemically stabilized” perfluorosulfonic acid membranes, and the development of membranes with backbone structures with improved chemically stability, such as polysulfone-like structures, e.g., US 20080275146. The common approach to addressing (i) and (ii) is through “support member” or “compression member” designs, e.g., U.S. Pat. No. 6,500,319, U.S. Pat. No. 6,855,450, U.S. Pat. No. 7,217,472, and US 20090114531. New approaches to cell design in general could further address (i) and (ii), in particular addressing requirements for operation with significant differential pressure across the membranes as well sufficiency and uniformity of cooling to all cells, especially in large cell stacks.
Scale up to not only large numbers of cells, but also larger active cell areas is required to meet the requirements of both current and potential emerging large scale industrial applications of hydrogen. Kondoh et. al. [J. New Mat. Electrochem. Systems 3 (2000) 61-65] reported limited testing of a PEM water electrolyser cell stack with a larger active cell area of 2,500 cm2, but only 10 cells; the feasibility of scale-up to the target of 300 cells remains unknown. Clearly, a design amenable to larger active cell area that also is inherently scalable with regard to the number of cells per cell stack would be advantageous.
As used herein, the terms “half cell”, “half electrolysis cell” and equivalent variations thereof refer to a structure comprising one electrode and its corresponding half cell chamber that provides space for gas, or gas-liquid (water) flow out of the half cell. The term “cathode half cell” refers to a half cell containing a cathode, and the term “anode half cell” refers to a half cell containing an anode.
As used herein, the terms “cell”, “electrolysis cell” and equivalent variations thereof refer to a structure comprising a cathode half cell and an anode half cell. A cell also includes a membrane, typically located between, and integral with, the cathodes and anodes. A membrane therefore defines one side of each half cell. The other side of each half cell is defined by an electronically conducting solid plate, typically comprised of metal, carbon, carbon-polymer composite, or combinations thereof, and generally known as a bipolar plate. The functionality of the bipolar plate is to maintain the fluids in adjacent half cell chambers of adjacent cells separate, while conducting current electronically between adjacent cells. Each half cell chamber also contains an electronically conducting component generally known as a current collector or current carrier, to conduct current across the half cell chamber, between the electrode and the bipolar plate.
Practical PEM water electrolysers utilize a structure comprising multiple cells, generally referred to as a “cell stack”, in which the cells typically are electrically connected in series. A cell stack typically consists of multiple cells, with bipolar plates physically separating but electrically connecting adjacent cells. One approach to cell stack construction is to use structural plates or “frames” to form the cell stack body, e.g., as exemplified in U.S. Pat. No. 6,500,319. As used herein, the term “structural plate” refers to a body which defines at least one half cell chamber opening. A cell stack typically is constructed using a series of structural plates to define alternately cathode and anode half cell chambers for fluid (gas, or gas-liquid mixtures and liquid) flow. The structural plates also hold functional components, which may include, for example, MEA's, electrode backing layers (separate, or as part of the MEA's), current collectors, and bipolar plates, in their appropriate spatial positions and arrangement. The series of structural plates and functional components typically constitutes a filter press type structure, including end pressure plates. In an alternative approach to cell stack construction, the structural plate, current collector and bipolar plate functionality can be combined in the bipolar plates, in which case, the bipolar plates further comprise stamped, machined or molded grooves or passages for fluid flow.
The cathode half cell chambers can be operated “dry” or “wet”: in the former case the cathode half cell chambers contain substantially only hydrogen gas (saturated with water vapour, and with any condensed water) during operation; in the latter case the chamber contains a gas-liquid mixture (hydrogen-water) during operation. The anode half cell chambers typically are operated “wet” (in order to supply feed water to the anodes), and contain a gas-liquid mixture (oxygen-water) during operation. The gas or gas-liquid mixture(s) typically are collected into manifolds at the exits of the half cell chambers. The gas-liquid mixtures must be treated in degassing vessels, which serve to separate the respective gases from the entrained electrolyte. The terms “electrolyser module” or “electrolyser” refer to a structure comprised of an electrolyser cell stack and its associated degassing vessels or chambers.
Most practical PEM water electrolyser modules today utilize “dry” cathode half cell chambers, and “wet” anode half cell chambers. Further, typically the anode side pressure is near-atmospheric, while the cathode side pressure is significantly higher, e.g., at least 100 psig. This type of system and operating approach is simple, low cost and minimizes exposure of circulating water to metallic parts, since water circulates only at low pressure, enabling the use of plastic gas-liquid separation vessels, circulation pumps, and interconnecting piping. This in turn minimizes contamination of the water by metal ions (which would “poison” the proton exchange membranes, reducing their conductivity), and water purification system requirements. However, this operating approach also results in significant differential pressures across the membranes, stress on the membranes, and potential durability and lifetime issues due to creep effects. The maximum differential pressure across the membranes generally is limited to 300 psi (depending on the membrane thickness, reinforcement, and cell configuration); consequently, typical hydrogen side pressures have been limited to significantly less than 300 psi in practice for “stand alone” cell stacks; that is, cell stacks without external pressure supporting structures or vessels. Furthermore, poor durability with thin membranes has been a barrier to realization of the excellent cell performance potential of PEM water electrolysers.
If higher hydrogen pressures are desired, then the typical approach is to place the cell stack inside a pressure vessel. The pressure vessel typically is filled with water, and is commonly used as the oxygen gas-liquid separation vessel, allowing for natural fluids circulation, without a mechanical pump, and ease of equalization of pressures outside the cell stack and in the anode half cell chambers, while maintaining flexibility in cathode side operating approach. The hydrogen (cathode) side can be operated dry or wet, and at the same pressure as the oxygen (anode) side or at a different pressure than the oxygen side. Of course, the requirement for a pressure vessel is a disadvantage, especially if PEM water electrolysers with higher gas production capacity are to be considered.
Cooling of the cell stack can be accomplished via cooling plates (plates with internal passages for coolant circulation) interspersed along the length of the cell stack; however, this approach adds complexity and cooling is potentially uneven, increasing the potential for development of hot spots.
Cooling of the cell stack also can be accomplished by cooling the circulating water, for example by a heat exchanger or by cooling elements in the gas-liquid separation vessel(s). Cooling can be expected to be most effective with “wet” cathode half cell chambers. With “dry” cathode half cell chambers, the anode side cooling also is relied on to remove a significant portion of the cathode side heat, but still provides more direct cooling of each and every MEA.
In order to address the shortcomings of known practical electrolyser modules, what is needed is a simple, cost-effective design that minimizes associated mechanical connections and assembly, while addressing the following aspects which have been lacking in known PEM water electrolyser designs: (i) significantly larger scale; (ii) inherent scalability (i.e., freedom to vary the number of cells over a wide range to meet a wide range of gas production capacities, including very high gas production capacity); (iii) simple operation without significant differential pressures across the MEA's, or alternatively, simple operation at higher hydrogen side pressure, without using an external pressure vessel or external structural supports; and, (iv) uniform and self-adjusting cooling of each and every cell in the electrolyser. Such a design, especially when further designed to provide a wide range of gas production capacity per cell, would be especially useful when connected to a source of electricity with variable output power, for example, a wind farm or a solar array.